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I met a guy that cut them in half and converted them into charcoal grills. Another guy collected them, brought them to the woods, and shot the hell out of them. I, like jtrammel, leave them for the scrappers.

Are you aware that the cylinders have convenient little disc/dome things that you can hit with a wrench or other metal object and then render the tank incapable of holding pressure? You don't need to use a torch to make the tank unusable, which is all you need to do...

I have a friend who cut a bunch of them up, silver-soldered them back together to make one long tank, then silver-soldered pneumatic fittings to one end. Made himself a nice air tank for his garage...

Converting them to air tanks is a dangerously bad idea. Air contains moisture, which rusts the inside of the cylinder.

Aren't normal air compressors just steel also? So they would also rust inside like a refrigerant tank would?

Are you aware that the cylinders have convenient little disc/dome things that you can hit with a wrench or other metal object and then render the tank incapable of holding pressure? You don't need to use a torch to make the tank unusable, which is all you need to do...

Yes, I’ve punched through a few when I was in a hurry to rid my truck of an empty cylinder taking up space and move on to the next service call.

I tend to be “over the top” in my endeavors on anything that passes through my hands. So, for ME, cutting out the valve leaves the cylinder readily and visibly rendered EMPTY and UN-reusable, thus removing all temptations of the innocent. Plus…cutting it with a torch lets me play with fire. AND...destroying an otherwise fully intact disposable cylinder allows me to release my pent up aggressions.

That valve also serves as a one-way check valve to discourage their reuse. However, the internet is a tremendous resource for finding ways AROUND stuff…

Originally Posted by cmclifton

I have a friend who cut a bunch of them up, silver-soldered them back together to make one long tank, then silver-soldered pneumatic fittings to one end. Made himself a nice air tank for his garage...

I certainly hope he "welded" them instead of merely brazing them with silver-solder!

Originally Posted by cmclifton

Aren't normal air compressors just steel also? So they would also rust inside like a refrigerant tank would?

Heck, a PET plastic Coke bottle is rated to 150+PSI!

A disposable refrigerant cylinder is engineered to deliver refrigerant. When considering how manufacturers try to cut their costs, don’t you think those cylinders will be manufactured as cheaply as possible?

Those disposable refrigerant cylinders are relatively thin, non-rust-resistant tanks. (there's no anticorrosion lining or coating on the inside, since they're designed to hold moisture-free refrigerant). So, the walls on a "virgin" refrigerant cylinder aren’t nearly as thick as a recovery cylinder, a LP tank or an air tank.

Refillable air tanks and other pressurized vessels are designed and engineered to be rated to a specific pressure and have wall thicknesses to serve in that capacity. They're a bit more expensive for a reason and, like all reusable pressure vessels, are required to undergo hydrostatic testing after a few years of service and then be retired after so many years.

The little, el cheapo portable and rechargeable air tanks you can buy at the auto parts stores are date-stamped for a service life of five years. Then you dispose of them. THOSE tanks have a safety relief valve that releases when the safe limit is approached.

Considering how cheaply those “purpose-designed” compressor tanks can be had, using a tank that once contained refrigerant isn’t worth it.

A STORY TO SHARE WITH YOU

During my years in the industrious environment of the military’s aviation community, we were multi-tasked with collateral duties. At several of my assignments, my collateral duty was as a safety manager.
We had a tire shop. We didn’t do aircraft tires there, just tow tractor tires and the like. Solid rim, split rim and demountable-flange (like on 18 wheelers). For inflating any of those tire/wheel assemblies OFF a vehicle, we had to put them inside a tire cage (½“ thick steel walls), then monitor the pressure with a ten-foot standoff gage from outside of the cage (of course).
A Corporal that worked for me suggested that we do a demonstration for training purposes to amplify the importance of tire/wheel maintenance safety practices. So, we set up a demountable-flange tire & wheel assembly inside that cage to inflate it, but purposely DID NOT fully seat the wheel flange's locking ring.
We held a safety class on the topic inside of our shop. The tire cage was outside. The stand-off hose was routed from the cage, underneath a closed bay door and to the inside our shop where the class would be held.
With twenty guys in attendance, I commenced in my requisite dissertation on the subject. Nobody knew about our “intended” demonstration. While I conducted the class, Corporal Heinz proceeded to feed air to that tire outside while monitoring the pressure.
The tire was rated for 80 psi. With our “setup”, we weren’t even sure anything would happen. He was giving me hand signals on the progress of the tire pressure.
I was beginning to doubt this demonstration would produce any results after “six fingers” for 60 psi.
J-U-S-T as he was going to flash seven fingers -

BOOOOM!!!!!!!

The class jumped! Corporal Heinz and I jumped!
People came running from the maintenance offices. People came outside from the buildings across the street. Of course, the noise of that ring separating off the wheel was amplified by the steel walls of the tire cage. So it truly sounded like a bomb went off.
With all the unintended attention brought to the scene by all of these people wanting to know what happened, I was having my own “Awe S#/+!” moment.
Needless to say, the demonstration went off with a BANG. That wheel’s locking ring also left a permanent impression in the wall of the tire cage.

*
*
So, aside from a coke or a beer (), I maintain tremendous for respect for anything considered “contents under pressure” and thereby highly discourage repurposing disposable refrigerant cylinders as portable air tanks.

Recover whatever refrigerant is left in it, then cut a hole a few inches in diameter in the top. Now you have an interesting container for all sorts of stuff. Like giving it to your children along with some junk gauge sets so they can be "the A/C guy/girl" on Halloween.